Recently, portable devices such as a portable telephone, a notebook-sized personal computer, a personal digital assistant (PDA) and the like have been frequently used. These portable devices have been served for various purposes of use with the development of network system and software, and convenience for users have been heightened. Hereby, types of functional components on-boarded in portable devices such as a speaker, a microphone, a receiver, a vibrator, a camera, a liquid crystal display, a card memory, LSI, an infrared ray communication module and the like tend to increase in number.
To heighten portability and convenience of portable devices, downsizing and thinned thickness of them have been required. However, increase of functional components used in portable devices have disturbed downsizing of the portable device. Further, when a portable device is dropped, functional components in the portable device may be broken due to collision of the functional components with each other or collision of the functional components with a case body. Therefore, it has become important to arrange the functional components and to design spaces among the functional components while considering to prevent this breakage of the functional components. Moreover, to come into wide use portable devices more, it is indispensable to develop techniques for manufacturing and installing parts at low cost in portable devices.
In portable devices, various movable parts driven in response to electric signals are used. As these movable parts, for example, in portable telephones, there are an acoustic device, a vibrator, a camera zoom mechanism and the like. And, in a personal computer and PDA having a cooling system of liquid cooling type, there is a coolant-circulating pump. Further, a battery is used for a portable device as an energy supply source. Therefore, because an electromagnetic type driving source such as a direct current (DC) motor or a magnetic force to mechanical force converter is possible to be driven at low voltage such as 3 to 5 V and is obtained at low cost, the electromagnetic type driving source is used as a driving source for driving movable parts.
However, because an electromagnetic coil or a permanent magnet is used for an electromagnetic type driving source, it is technically difficult to downsize the driving source while keeping a driving performance of the driving source. For example, in a portable telephone, an electromagnetic speaker is used as an acoustic device generating an incoming sound. However, the electromagnetic speaker has a thickness of about 3 mm. When the electromagnetic speaker is further downsized and thinned, performance of the electromagnetic speaker is undesirably lowered. Therefore, it is generally considered that downsizing and thinned thickness are difficult.
Therefore, as driving sources for movable parts described above, a piezo-electric ceramic transducer using no electromagnetic function, having high electric-mechanical energy conversion efficiency and advantageous in downsizing and thinned thickness have attracted attention of users. A piezo-electric ceramic transducer is deformed when a direct current is applied to the transducer, and the piezo-electric ceramic transducer can move an object. Further, when an alternating current voltage of a desired frequency is applied to a piezo-electric ceramic transducer, the piezo-electric ceramic transducer is vibrated at the frequency. Therefore, the piezo-electric ceramic transducer can vibrate an object.
Various types piezo-electric ceramic transducers have been proposed as a mechanical driving source and are put to practical use. For example, in “Ultrasonic Electronics Vibration Theory—Basics and Applications—” (edited by Yoshiro TOMIKAWA, Asakura Publishing Co., Ltd., Feb. 20, 1998, pp. 104-131), a piezo-electric ceramic transducer warped and deformed due to a piezo-electric traverse effect and movement of the transducer are written in detail. An example of the piezo-electric ceramic transducer is shown in FIG. 1. FIG. 1 is a schematic view showing a conventional piezo-electric ceramic transducer. As shown in FIG. 1, conventional piezo-electric ceramic transducer 101 is structured by bonding two piezo-electric ceramic plates 103 and 104, which are polarized and have electrodes 105 on both principal surfaces, on both surfaces of thinned-plate shaped permanent elastic body 102, which is made of metallic material and functions as one of electrodes for applying voltages, with adhesives. Further, in this piezo-electric ceramic transducer 101, to apply a voltage to each of piezo-electric ceramic plates 103 and 104, a total of three lead wires 106 are connected to electrodes 105 and permanent elastic body 102, respectively. Such piezo-electric ceramic transducer 101 having a bonded structure of two piezo-electric ceramic plates 103 and 104 is generally called a bimorph type.
When a voltage is applied to piezo-electric ceramic plates 103 and 104 of piezo-electric ceramic transducer 101 while devising polarity of the piezo-electric ceramic plates 103 and 104 so as to contract one of piezo-electric ceramic plates 103 and 104 in a longitudinal direction thereof and to expand the other in a longitudinal direction thereof, piezo-electric ceramic transducer 101 is bended and displaced in a thickness direction thereof. This movement is generated in the same manner regardless of whether a flat surface of piezo-electric ceramic transducer 101 has any shape such as a rectangle, a square, a circle or the like. Further, when an alternating current voltage is applied to this piezo-electric ceramic transducer 101, piezo-electric ceramic transducer 101 is vibrated in a thickness direction thereof. When a frequency of the applied alternating current voltage is adjusted, resonance is observed in a natural period of piezo-electric ceramic transducer 101 determined by size, material and the like of piezo-electric ceramic transducer 101, and amplitude of piezo-electric ceramic transducer 101 is maximized.
Further, as described above, a battery having an output voltage of 3 to 5V is often used for a portable device as an energy source. However, it is generally known that piezo-electric ceramic transducer has a higher operational voltage as compared with that of an electromagnetic type driving source. To solve this problem, a multilayered type piezo-electric ceramic vibrator has been proposed. This vibrator is produced by alternately stacking thinned piezo-electric ceramic plates and electrodes, and can be driven at low voltage by heightening intensity of applied electric field. For example, as shown in FIG. 2, piezo-electric ceramic vibrator 111 obtained by alternately stacking electrode layers 115 and piezo-electric ceramic layers 113 has been disclosed in “Mechanical Quality Factor of Multilayer Piezoelectric Ceramic Transducers” (Yashuhiro SASAKI et al., Jpn. Appl. Phys. Vol. 40(2001), Part 1, No. 5B, pp. 3549-3551, May 2001). In FIG. 2, electrode layers 115 facing each other in each pair with one piezo-electric ceramic layer 113 between are wire-connected to first external electrode 116 so as to be electrically in parallel to each other. Further, second external electrodes 117 are formed on almost whole area of both principal surfaces of piezo-electric ceramic vibrator 111, and piezo-electric ceramic vibrator 111 is structured so as to lead out electric terminals (not shown) from these electrodes.
However, to actually apply a piezo-electric ceramic transducer, using a piezo-electric traverse effect and performing bending and displacing movement, to a portable device, there are problems described hereinafter.
First, to operate a bimorph type piezo-electric ceramic transducer shown in FIG. 1, it is required to lead out at lest one electric terminal lead wire from each of both principal surfaces of the piezo-electric ceramic transducer in a thickness direction of the piezo-electric ceramic transducer and one electric terminal lead wire from a permanent elastic body, that is, a total of three lines.
For movement of a mechanical driving source in a portable device, frequent movements and long-time stability are required. However, as the number of electric terminal lead wires becomes large, a possibility of disconnection is increased. Further, when a piezo-electric ceramic transducer is vibrated, a problem arises in that noises are generated due to acoustic radiation from electric terminal lead wires.
Further, because electric terminal lead wires are connected to both principal surfaces of a piezo-electric ceramic transducer in a thickness direction thereof, these electric terminal lead wires disposed on both principal surfaces of the piezo-electric ceramic transducer in the thickness direction becomes an obstacle. Because of these problems, not only a joining position to an object, to which displacement or vibration of piezo-electric ceramic transducer should be transmitted, is restricted, but also it is required to secure both an occupation space and a displacement movement space for electric terminal lead wires. Therefore, features of downsizing and thinned shape in a piezo-electric ceramic transducer are undesirably lost. Further, because electrodes are formed on both principal surfaces of a piezo-electric ceramic transducer in a thickness direction thereof, in case of the object made of metal, electric energy applied to the piezo-electric ceramic transducer is leaked to the object. Therefore, lowering of reliability caused by corrosion of the object is feared, and adverse influence on human bodies and/or noise mixture to other parts based on transmission of electric energy to a structured element such as a housing of a portable device and the like through the object are feared.
Secondly, when a mechanical driving source is installed in a portable device, a small size and a thin shape not obtained in the past are required. In this case, piezo-electric ceramic plates (piezo-electric layers) constituting a piezo-electric ceramic transducer and a permanent elastic body are inevitably made in a thin plate shape. In a sintering process for piezo-electric ceramic plates, when a thin plate is sintered, sintering distortion is generated in the thin plate. This sintering distortion generates errors in size of electrodes and degradation of characteristics of piezo-electric material. Further, in a process for joining a permanent elastic body to a piezo-electric ceramic plate, mismatching is generated on a surface of the piezo-electric ceramic plate joined to the permanent elastic body. Because of irregularity in the joining process, reliability of the piezo-electric ceramic transducer is lowered. To lessen the sintering distortion, a pre-body of a piezo-electric ceramic plate is formed in a size larger than a final size of the piezo-electric ceramic plate, and this pre-body is sintered. Thereafter, grinding and polishing are performed for the sintered body, so that the plate having a desired size is obtained. Due to these processes, manufacturing cost of the piezo-electric ceramic plates is increased.
Further, because each piezo-electric ceramic plate is a thin plate, defects such as breakage, crack and/or the like are easily generated in the piezo-electric ceramic plate during handling in a manufacturing process. This induces lowering of yield. As a result, manufacturing cost is increased.
Thirdly, there is a problem in a frequency band when a piezo-electric ceramic transducer is vibrated. Now it is considered to obtain vibration of a desired frequency by using a piezo-electric ceramic transducer. Amplitude of a piezo-electric ceramic transducer is increased at a resonance frequency of the piezo-electric ceramic transducer, but the amplitude is extremely attenuated at a frequency lowered than a fundamental resonance frequency of the piezo-electric ceramic transducer. That is, in a case where the fundamental resonance frequency can be adjusted according to a required frequency, convenience is improved. As an example, in case of application of a piezo-electric ceramic transducer to a speaker, when the piezo-electric ceramic transducer has two or more resonance frequencies within a region of 300 Hz to 1 kHz in an audible band, levels of sound pressure in a low sound region are equalized. Therefore, a speaker having preferable tone quality can be substantiated such that the speaker reproduces faithfully sound of a low sound region, though it is difficult to reproduce sound of the low sound region by using a conventional speaker using a piezo-electric ceramic transducer.
However, a fundamental resonance frequency is uniquely determined based on physical properties and shape of material constituting a piezo-electric ceramic transducer, so that it is difficult to arbitrarily change a resonance frequency in a single piezo-electric ceramic transducer. Therefore, to have two or more resonance frequencies, it is required to manufacture a plurality of piezo-electric ceramic transducers having different shapes. As a result, because manufacturing cost of such piezo-electric ceramic transducers are increased, possibility of application of these piezo-electric ceramic transducers to a mechanical driving source of a portable device is lowered in practical use.